Apparatus of this general type is well known. U.S. Pat. No. 4,098,118 discloses a measured value recording device wherein two radially inwardly directed magnetic coils are bolted to the internal surface of a relatively thin cylindrical jacket at diametrically opposite positions therein. There is further provided in that device a plastic tube through which flows the medium being measured. The outside diameter of the tube is substantially smaller than the inside diameter of the jacket and the tube carries at one end a flange limiting at one end the cavity between the tube and the jacket. Molded onto the outer jacket of the likewise relatively thin plastic tube are radially outwardly directed sockets defining bore holes for taking up the electrodes.
After affixing the magnet coils inside the jacket of that prior device, the plastic tube provided with the electrodes is inserted into the jacket. Then the relatively broad cavity between the tube and jacket is filled with a grout of plastic material, whereby the magnet coils and the measuring electrodes are embedded in the grout mass. This construction has all of the disadvantages that arise with the casting, or injection molding, respectively, of plastic masses. Furthermore, after the grouting, the measuring electrodes and coils are inaccessible so that in case of a defect in one of these components, as a rule, the whole device has to be replaced.
In another conventional construction described in U.S. Pat. No. 3,924,466 there are provided particularly flat magnet coils which consist of thin, flexible plastic foils forming thin current conductor paths. These plastic foils are affixed to the outer jacket of a plastic tube and inserted with this tube in a cylindrical jacket of ferromagnetic material. Diametrically opposite measuring electrodes are positioned in radial bore holes extending through the jacket, the flat magnet coils and the plastic tube. There are further provided in that device connector bolts, extending through the jacket for the current conductor paths of the magnet coils. For a short-circuit of the magnetic field there are either wound several layers of a metal foil or a plastic tube made out of Duroplast is chosen that is heated after insertion into the jacket in order to press the magnet coils tightly against the jacket which then acts as a magnetic field short-circuit. That prior design permits mechanical assembly of the component parts of the device only in connection with special flat foil-like magnet coils; it is wholly unsuitable for installing conventional magnet coils.
In case there is very little space available between the flanges of the tubing for taking up the measured value recording device then German OS No. 15 73066 proposes an especially narrow construction wherein the measured value recording device is formed in the shape of a flange consisting of an insulating material which contains the electrodes and radially inserted magnetic poles, whereby a curved yoke around the flange is provided with a magnet coil. That prior device produces an unfavorable distribution of the magnetic field. Also it requires excessive space in radial direction for accommodating the magnetic components. That is, it often extends out beyond the bounds of the fluid tubes to which it is connected.